In this context an extreme state is to be understood to mean such a state that the pixel is substantially completely transmissive or non-transmissive. This state is determined by the nature of the ferro-electric display device which will be described in greater detail hereinafter. The choice of the amplitude of the auxiliary signal is also decisive of the rate at which the liquid crystal switches to this extreme state.
The method mentioned above are used, for example, in display devices for display screen or television uses. Notably for television uses and non-volatile memory display devices the use of ferro-electric liquid crystal materials seems to be attractive because much faster switching times can be realized therewith than with effects based on nematic liquid crystal materials.
Other advantages are that the viewing angle dependence is smaller (because generally a smaller thickness is used and because of the orientation of the molecules in faces parallel to the walls) and that the contrast is greater.
A method of the type mentioned in the opening paragraph is described in European Patent EP No. 0,197,742. In this method the presentation of data signals synchronously with a selection signal is preceded by so-called blanking pulses which must bring the liquid crystal to an initial state. In order to inhibit degradation of the ferroelectric material, the pulses used periodically reverse their sign.
The method described is used in a display device having a so-called passive matrix; and the signals (selection signals, data signals, blanking pulses) are directly presented to the row and column electrodes. The state of a pixel is determined by the line and data voltages. Since data voltages are presented to the column electrodes also when certain pixels are not selected, voltages, possibly in combination with cross-talk signals, may be produced so that the desired transmission state (in the example of European Patent EP No. 0,197,742 transmissive or nontransmissive) is not achieved or is lost.
The above-cited Patent Application proposes to solve this problem by providing the ferro-electrical effect with a certain threshold. Such thresholds cannot be realised or can hardly be realised in practice without contrast loss occurring in applications using larger numbers of lines. This Patent Application also refers only to switching between the states "0" and "1" and the possibility of introducing grey scales is not dealt with in the description of the devices shown.